Avril is a novice in a convent of "Baptistine" sisters, a monastic order which was officially dissolved by the end of the nineteenth century but that is kept alive by Mère Marie Joseph, the sadistic superior. The rule she imposes on the nuns is particularly strict but this is all Avril has ever known since she was born, for she was an abandoned child raised by the nuns with a view to making one of them. While Avril is on retreat, locked in for a fortnight in a chapel prior to taking her vows, Soeur Bernadette, a sympathetic sister, discloses a secret to her: she has a twin brother and she encourages her to go looking for him...